Nanoparticles of different shapes have attracted a great deal of attention due to their potential application in catalysis, photonics and other fields such as nanomedicine.
Nanoparticle shape is related to the properties of nanoparticles, and may influence their selectivity in catalytic reactions and their interaction with light and living matter. In particular, platinum nanoparticles have been studied for their unique catalytic properties.
Elemental platinum has a face-centered crystal structure and low index facets [100], [111], [110] are common on platinum nanoparticles. Cubic particles are made up entirely of [100] facets, octahedral particles of [111] facets, whilst cuboctahedra are made of both. During wet chemical reduction (WCR) for nanoparticles synthesis, the autocatalytic growth stage is the moment in which the shape of the nanoparticles is determined. Under pure thermodynamic growth conditions, a platinum nanoparticle will grow as a sphere or with an amorphous shape in order to minimize its surface energy. However, when the reaction occurs under kinetic control, due to reduced reduction rate and/or the presence of shape-directing agents, cubic, tetrahedral, octahedral shapes can be created.
The use of preferentially oriented platinum nanoparticles as catalysts for a variety of chemical reactions has proven to be highly advantageous and represents a major breakthrough for catalysis (M. Duca, P. Rodriguez, A. Yanson and M. M. Koper, Top Catal., 2014, 57, 255-264). For example, in the case of nitrite reduction in alkaline media, the Pt [100] facet has shown to be the most active surface, and is able to reoxidize ammonia to give nitrogen [M. T. M. Koper, Nanoscale, 2011, 3, 2054-2073; M. Duca, M. C. Figueiredo, V. Climent, P. Rodriguez, J. M. Feliu and M. T. M. Koper, J Am Chem Soc, 2011, 133, 10928-10939).
It has also known that by using a Pt [100] electrode as a catalyst for nitrite reduction, it is possible to obtain direct conversion of nitrogen dioxide to nitrogen. This voltammetric feature is extremely sensitive to the quality of the [100] domains and it has been observed that the introduction of controlled defects of any symmetry causes a rapid drop in the selective electrocatalytic conversion to nitrogen. The main voltammetric feature (nitrite reduction to ammonia) is also affected by the loss of surface order. Therefore, this reaction requires nanoparticles with specific characteristics: cuboid nanoparticles with large, well-ordered [100] domains.
Shaped platinum nanoparticles have been obtained by wet chemical reduction using additives and capping agents to control shape (shape directing agents). The presence in the autocatalytic growth stage of shape directing agents such as polymers, surfactants, organic ligands, and ionic salts plays a crucial role in the formation of shaped nanoparticles. Generally these species act by either binding to specific facets to promote asymmetric growth or by altering reduction kinetics so that growth proceeds under thermodynamic or kinetic control. For example, U.S. Pat. No. 8,257,465 describes a method for controlling the shape of metal nanoparticles by using bromide as a shape-directing agent. Bromide is adsorbed onto the surfaces of a seed crystal and is then treated with an oxidizing agent, which oxidatively etches one surface of the seed crystal. Then, in the presence of metal precursor compound and a reducing agent, the exposed surface is able to grow to produce a nanostructure having [100] and [110] facets. It is reported that in the absence of bromide, only cuboctahedrons were produced.
Slower reduction conditions are favoured for obtaining shaped nanoparticles, as they allow more time for the shape-directing agents to interact at the surfaces of the metal particles, making them more effective at shape direction. A lower rate of reduction also favours the formation of fewer seeds and leads to a higher precursor-to-seeds ratio that aids shape formation (J. Yin, J. Wang, M. Li, C. Jin and T. Zhang, Chemistry of Materials, 2012, 24, 2645-2654). Gumeci et al. (J. Phys. Chem. C 2014, 118, 14433-14440) describe a method for the preparation of metal nanoparticles using Pt(acac)2 as the precursor in DMF containing water. The heating rate and reaction temperatures were selected to promote rapid nucleation of seed crystals followed by a period of slow growth to achieve shape-controlled nanoparticles. It was found that increasing the water content beyond 20% (by volume) resulted in agglomeration and loss of shape control.
Another parameter that may influence shape growth is the stabilization of a facet through the use of additives. Chemical species such as ligands and polymers have been used to selectively stabilize specific facets by slowing the growth of these facets, thus making them more abundant on the surface of the final nanoparticle. Unfortunately, ligands and polymers also affect the catalytic properties of the material, and surfactants, such as tetradecyl trimethyl ammonium bromide have shown, to be non-beneficial to catalytic activity (A. Miyazaki, I. Balint and Y. Nakano, Journal of Nanoparticle Research, 2003, 5, 69-80). Oleylamine has also shown to be highly detrimental to the catalytic activity of carbon monoxide oxidation, due to the poisoning properties of the amine group (J. N. Kuhn, C.-K. Tsung, W. Huang and G. A. Somorjai, Journal of Catalysis, 2009, 265, 209-215). Furthermore, although it is possible to partially or completely remove the organic coatings on the surface of the nanoparticles by UV-ozone treatment, thermal annealing or acetic acid washing, these treatments are time-consuming, costly, do not guarantee complete removal. Organic coatings can interfere with the catalytic properties of the nanoparticles.
A method of controlling the shape of metal nanoparticles without using shape-directing agents is described in patent application WO2014/162308 A2. Here it was found that fast heating and cooling rate of a reaction solution influenced the formation of nanoparticles with [100] facets. In order to control the temperature of the reaction solution, a flow system was employed whereby the reaction solution could be passed to a reaction zone and heated for a certain time before being allowed to flow to a cooling zone. The method did not include the use of metal seed nanoparticles.
Despite the advances in the synthesis of nanoparticles having controlled shape and size, there still exists the need in the art for simple, eco-friendly methods of obtaining preferentially shaped nanoparticles that are free of contaminants.